Filtration media are widely used in industry in various forms for the filtration and removal of particles from fluid such as air, hydraulic fluid, etc. In response to increasing demands from industry and government for higher degrees of filtration, filtration media have been developed having smaller fiber size for trapping a smaller class of particles from the fluid. An important class of filtration media is non-woven fibrous materials. This type of filtration media generally consists of a matrix or mass of fine diameter fibers, with the fibers sufficiently close to each other and in a sufficiently thick layer so that the interfiber spacing or effective pore size is small enough to trap the particles of the desired size range for the application of the filter. The filter media are then fabricated into the desired form for the particular application, i.e., panels, pleated cartridges, flat disks, and canisters, etc., as is generally known in the art.
In making non-woven fine fiber filter media, a variety of materials have been used, including fiberglass, metals, ceramics, and a wide range of polymer materials. Fibers have been formed by a variety of techniques including forcing the materials through fine capillaries or openings, either as melted material or in a solution that is subsequently evaporated. Formation of a filtration layer of solution-blown polymeric microfibers is disclosed in U.S. Pat. No. 4,011,067. Fibers have also been formed by "spinning" fibers through the use of "spinnerets" of the type long used in the textile industry for the manufacture of synthetic fiber, and electrostatic spinning. In electrostatic spinning fibers are formed as they leave a nozzle or capillary and are attracted to a collection zone by the high voltage electrostatic field contained in the apparatus. (See U.S. Pat. No. 4,143,196)
Developments in the art of producing fine fibers have resulted in the production of fibers having micron or submicron diameters, and this in turn has permitted the formation of filter media fiber matrices capable of filtering submicron particles more effectively.
Thin non-supporting layers of filtration media have been deposited on self-supporting base layers forming multilayered filter media. One such multilayered filter media is disclosed in previously referenced U.S. Pat. No. 4,011,067.
To protect and further support the thin filtration layer a top protective layer may be provided. In existing multilayered filter media this top layer is generally much thicker than the filtration layer and is laminated or affixed with adhesive over the thin filtration layer. Cover layers of this type are susceptible to release of fibers to the surrounding environment; add undesirable restriction to fluid flow through the media; add undesirable thickness to the resulting media and require the application of adhesive and/or pressing together of the cover layer and filtration layer.
In certain environments, such as the computer field in general, and hard or "Winchester" disk drives in particular, an extremely high degree of filtration is required. Particulate matter which becomes lodged between the disk and the magnetic read/write head which "flies" only a few microns or less above the disk on an air cushion can destroy the disk drive unit; therefore, flaking and chafing of the filter media must be absolutely minimized. Realizing that hard disk drive units are sealed for life, providing a filter media having a smooth outer surface which does not exhibit flaking or sloughing of media fibers is highly desirable.
Therefore, a need exists for a multilayered fiber media having a smooth, thin self-adhering protective outer layer which improves durability and performance without increasing the flow restriction of the overall media.